Gram staining is a method of differentiating bacterial species into two large groups which are the Gram-positive and Gram-negative bacteria, which was devised by Danish physician H. C. J. Gram (1853-1938) in 1884 not for the purpose of distinguishing one type of bacterium from another but to enable bacteria to be more visible in stained sections. These bacterial groups are generally different in various aspects including sensitivity to chemical agents, nutrients necessary for their growth, response to physical and chemical stimuli, toxins produced thereby, and lesions caused thereby. Representative among Gram-positive bacteria, characterized by a purple color stain, are Staphylococcus spp., Pneumococcus spp, Mycobacterium leprae, Corynebacterium diphtheria, Clostridium tetani, Bacillus anthracis, and Actinomyces. Most of these bacteria are leading pathogens responsible for serious disease such as diphtheria, tuberculosis, pneumonia, etc.
For example, Staphylococcus aureus, a Gram-positive bacterium, is a pathogen which causes various diseases including food poisoning, impetigo, cellulitis, scalded skin syndrome, mastitis, bacteremia, sepsis, staphylococcal pneumonia, endocarditis, heart failure, osteomylitis, Staphylococci sepsis, circulatory collapse, and toxic shock syndrome. In the United States, staphylococcal infection is diagnosed in more than two million people every year and accounts for the death of more than ninety thousand people.
Most bacterial infections are readily cured by antibiotics such as penicillin, etc., but some have recently been reported to be difficult to treat. Bacteria become resistant to antibiotic drugs through a spontaneous DNA mutation or the uptake of foreign DNA so that they are able to survive exposure to an antibiotic. This situation is called “antibiotic resistance” and the bacteria with antibiotic resistance are called “drug resistant bacteria.” In addition, drug-resistant bacteria show multidrug resistance in which, for example, only 10% and 50% of them can be regulated with penicillin and methicillin, respectively.
Antibiotic resistance has been found over a broad spectrum of Gram-positive pathogenic bacteria including Staphylococcus, Streptococcus, Entracoccus, Bacillus and Listeria. Of them, MRSA (Methicillin-Resistant Staphylococcus Aureus), VRSA (Vancomycin-Resistant Staphylococcus Aureus), MRE (multiple-drug resistant enterococci) and VRE (Vancomycin-Resistant enterococci) attract social interest because they are more dangerous.
For example, MRSA is treated with vancomycin, more potent than methicillin, but some show resistance to the drug, as well. Because such resistant pathogens are poor in susceptibility to drugs, it is often necessary to apply a large dose of antibiotics which is, however, noxious to the patient. In recent years, the increasing spread of drug resistance by S. aureus has been recognized as serious threat to the health of mankind. Therefore, there is a pressing need for a novel antibiotic that works through a new mechanism that confers a potent therapeutic effect even at a small dose thereof on an infection of the drug-resistant S. aureus. 
Classically, new antibiotics have been developed by chemically modifying bioactive compounds produced by microorganisms. With the great progress of genomic and drug discovery techniques such as high-throughput screening, many new drug targets are secured from which various drug candidates have been separated. The time and cost which it takes for such new drug candidates to undergo clinical trials and to acquire the approval of the FDA is currently acting as the main hindrance to the development of novel drugs. Hence, if found, new uses of commercially available drugs which have passed clinical trials and have had their safety verified in vivo will provide the advantage of significantly reducing the time and cost required to bring novel drugs to market.
Given this background, the present inventors have searched for finding a drug that works in a manner different from already known antibiotic mechanisms in the art in order to overcome the problem with multidrug resistance and, in addition, for finding a compound that exhibits antibiotic activity from among drugs currently used in clinics in order to make it easier to develop a novel antibiotic agent.